In conventional, in a brake apparatus of a dump truck traveling on a dirt road, a brake is actuated by pedaling a brake pedal, transmitting an air from a brake valve to a chamber cylinder in both of front tire wheels and rear tire wheels, converting the air into an oil pressure by the chamber cylinder, and operating a wheel cylinder on the basis of the oil pressure.
Further, in recent years, in order to improve a reliability and simplify a structure in the brake apparatus, there has been employed a full hydraulic brake system in which both of the front and rear tire wheels are controlled by the oil pressure.
In FIG. 5, a brake apparatus 61 employs a plurality of hydraulic accumulators 62a, 62b and 62c (hereinafter, refer to a hydraulic source 62) as a hydraulic source. A pressurized oil of the hydraulic source 62 is fed to a front tire wheel brake 64 and a rear tire wheel brake 65 via a foot type brake valve 63, thereby respectively controlling the tire wheels. The hydraulic source 62 receives a pressurized oil of a hydraulic pump 66 driven by a drive source 68 and accumulates the pressurized oil with a predetermined pressure, and unloads the pressurized oil in the hydraulic pump 66 by an unloading apparatus 67 in the case that the pressure reaches the predetermined pressure.
The foot type brake valve 63 is constituted by a front tire wheel brake valve 63a and a rear tire wheel brake valve 63b, and is structured such that when a pedal 63d is operated, the rear tire wheel brake valve 63b feeds the pressurized oil in the hydraulic source 62 to the rear tire wheel brake 65, and the front tire wheel brake valve 63a feeds the pressurized oil in the hydraulic source 62 to the front tire wheel brake 64, thereby braking respectively. An emergency and parking brake 70 is additionally provided in the rear tire wheel brake 65.
In the case that the emergency and parking brake 70 is used as a parking brake, a parking braking pressure Sp in a rear tire wheel cylinder chamber 72 is controlled by an electromagnetic parking brake valve 71 (hereinafter, refer to a parking valve 71), whereby a vehicle is parked. That is, the parking valve 71 is switched to a position b by a parking switch (not shown) at a time of parking, thereby shutting the pressurized oil of the hydraulic source 62, and returning the pressurized oil of the rear tire wheel cylinder 72 to a tank 73 so as to make the parking braking pressure Sp zero. Accordingly, a rear tire wheel brake 74 is pressed by the rear tire wheel spring 75 so as to be crimped, thereby parking the vehicle.
At a time of traveling, the parking valve 71 is switched to a position a, the pressurized oil of the hydraulic source 62 is supplied to the rear tire wheel cylinder chamber 72, and the parking braking pressure Sp is made high. Therefore, the parking braking pressure Sp within the rear tire wheel cylinder chamber 72 presses back the rear tire wheel spring 75 so as to open the rear tire wheel multi-disc brake 74 and move the vehicle.
On the other hand, in the case that the emergency and parking brake 70 is used as an emergency brake, an emergency braking pressure Cp in the rear tire wheel cylinder chamber 72 is controlled by a foot type emergency brake valve 77 (hereinafter, refer to an emergency valve 77), whereby the vehicle is emergency braked. That is, when a foot type pedal 78 is pedaled at a time of emergency, the emergency valve 77 reduces the pressurized oil of the hydraulic source 62 in response to the pedaling amount (amount of operation), and outputs the pressure to the rear tire wheel cylinder chamber 72 as the emergency braking pressure Cp. In this case, a brake valve spring 77a and a pilot pressure chamber 77b are additionally provided in one end side and another end side of the emergency valve 77, respectively.
At a time of traveling when the foot type pedal 78 is not pedaled, since the brake valve spring 77a presses a spool (not shown) against the pressurized oil introduced into the pilot pressure chamber 77b, the emergency valve 77 is at a position c for communicating the hydraulic source 62 with the rear tire wheel cylinder chamber 72. Accordingly, the emergency valve 77 outputs the pressurized oil in the hydraulic source 62 to the rear tire wheel cylinder chamber 72 via the parking valve 71 as a high emergency braking pressure Cp. The emergency braking pressure Cp presses back the rear tire wheel spring 75 so as to open the rear tire wheel multi-disc brake 74, thereby traveling the vehicle.
At a time of emergency when the foot type pedal 78 is pedaled, since a force of the pressurized oil introduced into the pilot pressure chamber 77b presses a spool (not shown) against the brake valve spring 77a, the emergency valve 77 is at a position d for communicating the tank 73 with the rear tire wheel cylinder chamber 72. Therefore, the emergency valve 77 reduces the pressurized oil of the hydraulic source 62 in response to the pedaling amount, and outputs the pressure to the rear tire wheel cylinder chamber 72 via the parking valve 71 as a low emergency braking pressure Cp. Since the rear tire wheel cylinder chamber 72 becomes a low pressure, the rear tire wheel spring 75 presses back so as to crimp the rear tire wheel multi-disc brake 74. In this case, since the rear tire wheel cylinder chamber 72 is exposed to the low emergency braking pressure Cp which is reduced in response to the pedaling amount, the crimping force of the rear tire wheel multi-disc brake 74 is adjusted, and it is possible to obtain the braking force at the emergency time in response to the pedaling amount.
In accordance with the structure mentioned above, all of the brake control is achieved by the oil pressure in the construction machine such as the dump truck or the like, however, in this case, the following problems are generated.
For example, when the foot type brake valve 63 bites foreign materials or the like thereinto and the operation thereof becomes hard, the emergency valve 77 is pedaled, whereby the emergency brake using the emergency and parking brake 70 provided in the rear tire wheel is operated.
At this time, only the parking brake provided in the rear tire wheel is operated in the emergency brake. Further, the emergency brake reduces the emergency braking pressure applied to the parking brake, and operates the mechanical type brake using the spring so as to brake. This is because in general, the mechanical type brake such as the rear tire wheel brake is not attached to the front tire wheel brake, and the front tire wheel brake can not be used as the emergency brake. It is advantageous that the emergency brake is divided into the front and rear tire wheels, however, since the conventional brake apparatus can not employ the emergency brake for the front tire wheel, the conventional brake is inferior to the case that the front and rear tire wheels are braked, in view of stopping the vehicle rapidly and stably.
Accordingly, it is desired to brake both of the front and rear tire wheels. In order to brake the front tire wheel by using a dry type single disc brake, it is necessary to continuously increase the emergency braking pressure inversely to the case of the rear tire wheel. That is, in order to brake both of the front and rear tire wheels, it is necessary to simultaneously carry out a process of increasing the emergency braking pressure in the front tire wheel and a process of reducing the emergency braking pressure in the rear tire wheel. On the contrary, there is a method of operating two pressure proportional control valves in a state of connecting the valves by a seesaw-shaped link. However, in the construction machine, a volumetric capacity and a weight of the apparatus are increased so as to secure a flow rate and a pressure, and there are many restrictions for mounting the apparatus on the vehicle, so that it is hard employ this method.
Further, it is possible to electrically control by using an electromagnetic proportional valve, however, in this case, incidental devices such as controllers, computers, sensors and the like are required, so that the control becomes complex. There is a risk that any problem is generated in a reliability, in the case that an electrical means is employed as a last means for the emergency stop.
It is possible to use the mechanical type brake together in the same manner as the rear tire wheel, however, a structure in this case becomes complex, the apparatus for the front tire wheel is large sized, and the vehicle becomes expensive.